A probe for the detection of methicillin-resistant Staphylococcus aureus

Identification of Staphylococcus aureus based on PCR amplification of species specific genomic 826 bp sequence derived from a common 44-kb Sma I restriction fragment

Primers were designed for polymerase chain reaction (PCR)-amplification of a genomic sequence specific to Staphylococcus aureus strains. The sequence corresponds to a part of the 44-kb Sma I fragment (fragment L on the S. aureus NCTC 8325 restriction map) which was found to be common to strains of the S. aureus species (Pantůcek et al 1996, International Journal of Systematic Bacteriology, 46: 216-222). The labelled 44-kb Sma I restriction fragment derived from S. aureus NCTC 8325-4 was hybridized to the Eco RI restriction patterns of genomic DNA from 13 strains representing different macrorestriction types of S. aureus subsp. aureus. This made it possible to reveal the 2052 bp Eco RI restriction subfragment and to demonstrate its presence in all the tested strains. From the sequence of this subfragment, primers were designed by means of which the 826 bp amplicons were obtained in all 216 tested strains of S. aureus. No hybridization and PCR-products were observed in 40 collection strains of other staphylococcal species and subspecies as well as in 45 clinical strains of coagulase-negative staphylococci. These results lead us to the conclusion that the use of the above primers makes it possible to identify rapidly and reliably S. aureus strains of various provenance and different genotypes.

Methicillin-resistant Staphylococcus aureus. Optimization of the agar screen plate method

Swab inoculation of oxacillin agar screen plates was compared with drop inoculation for detection of methicillin-resistant Staphylococcus aureus. The poor sensitivity of the swab method (59%) was related to heteroresistance of the S. aureus isolates. We recommend the drop method (100% sensitivity) because interpretation of results was significantly easier, making it more reliable.

Use of a DNA hybridization method to verify results of screening for methicillin resistance in staphylococci

Tests were performed by the disk diffusion method, agar dilution method and the E test to determine the susceptibility to methicillin and oxacillin of clinical isolates and control strains of Staphylococcus aureus (n = 106) and coagulase-negative species (n = 131). Results were compared with those of a dot blot DNA hybridization test, in which the mecA gene was detected using an oligonucleotide probe selected from the mecA gene. Among the Staphylococcus aureus strains the mecA gene was found in all but two strains inhibited by > or = 8 mg/l of methicillin and all but two strains inhibited by > or = 4 mg/l of oxacillin. A disk test using either 1 microgram oxacillin or 10 micrograms methicillin and a tentative resistance breakpoint of < or = 10 mm gave the best agreement with the hybridization test. For coagulase-negative staphylococci 34 of 35 strains inhibited by > or = 8 mg/l methicillin hybridized with the probe as well as 58 of 82 strains inhibited by 1-4 mg/l; 93 of 97 strains inhibited by > or = 0.5 mg/l oxacillin were also positive in the probe test. Using the 1 microgram oxacillin disk and a resistance breakpoint of < or = 10 mm good agreement was obtained between results of the disk diffusion and DNA hybridization tests. It is suggested that this genotypic method for detection of methicillin resistance is used as a reference method for routine methods.

Methicillin-resistant Staphylococcus aureus: a consensus review of the microbiology, pathogenesis, and epidemiology with implications for prevention and management

Methicillin-resistant Staphylococcus aureus (MRSA) has become a major nosocomial pathogen in community hospitals, long-term-care facilities, and tertiary care hospitals. The basic mechanism of resistance is alteration in penicillin-binding proteins of the organism. Methods for isolation by culture and typing of the organism are reviewed. MRSA colonization precedes infection. A major reservoir is the anterior nares. MRSA is usually introduced into an institution by a colonized or infected patient or health care worker. The principal mode of transmission is via the transiently colonized hands of hospital personnel. Indications for antibiotic therapy for eradication of colonization and treatment of infection are reviewed. Infection control guidelines and discharge policy are presented in detail for acute-care hospitals, intensive care and burn units, outpatient settings, and long-term-care facilities. Recommendations for handling an outbreak, surveillance, and culturing of patients are presented based on the known epidemiology.

Comparison of conventional susceptibility tests with direct detection of penicillin-binding protein 2a in borderline oxacillin-resistant strains of Staphylococcus aureus

Six selected strains of Staphylococcus aureus classified as borderline oxacillin-resistant, according to standard disk diffusion and microdilution susceptibility test methods, and seven methicillin-resistant and seven methicillin-susceptible control strains were examined for the presence of penicillin-binding protein 2a (PBP 2a) by fluorography and immunoblotting and for DNA hybridization with a mec-specific probe in a dot blot assay. Oxacillin agar screen tests with and without NaCl supplementation were also performed with all strains. PBP 2a was detected both by fluorography and by immunoblotting in all seven methicillin-resistant control strains and in none of the susceptible controls. PBP 2a was detected in two borderline strains. Results of agar screen tests performed without NaCl supplementation were completely concordant with susceptibility determined by PBP 2a and mec detection methods. Agar screening with NaCl supplementation was less accurate. These findings were confirmed with 20 additional borderline strains. Direct detection methods for the presence of PBP 2a or mec, the gene encoding it, allow accurate and definitive classification of borderline strains. Further efforts to develop a rapid, clinically useful, antibody detection system for PBP 2a are warranted.